1. Field of the Invention
The present invention relates to a deflection yoke capable of substantially reducing raster ringing (undulations of scanning lines on a TV raster screen) and hence obtaining a high reproduced image quality.
2. Description of the Prior Art
Deflection yokes capable of preventing raster ringing which is a source of degradation of image quality, have been proposed. Typical examples of the proposed prior arts of such are shown in Japanese Patent Application Laid-open Nos. 61-104544/1986 and 62-281242/1987, which will be described, first, with reference to FIG. 6 which shows an equivalent circuit of a horizontal deflection coil of such prior art.
According to the analysis of raster ringing in 61-104544/1986, distributed capacitances (unbalanced capacitances) C and C' in an upper coil 1a and a lower coil 1b of a horizontal deflection coil 1 are charged with a horizontal deflection current (horizontal pulse) and consequently currents I1, I2 and I1', I2' flow through the coils 1a and 1b. Ringing magnetic field oscillates by charging and discharging of the unbalanced capacitances C and C', resulting in raster ringing.
In order to prevent such raster ringing, 61-104544/1986 proposes a provision of intermediate taps 10c and 10d on the respective upper and lower coils 1a and 1b of the horizontal deflection coil 1 to which capacitors 11 and 12 whose capacitances Cx and Cx' are equivalent to the unbalanced capacitances C and C', respectively, are connected as shown in FIG. 7 to balance the distributed capacitance over the coils 10a and 10b to thereby reduce the raster ringing.
According to the analysis of raster ringing in 62-281242/1987, a cross-talk occurs from a horizontal deflection coil to a vertical deflection coil when a frequency of a horizontal deflection current is high and a distributed capacitance of the vertical deflection coil is charged with a voltage induced in the vertical deflection coil due to this cross-talk. Due to charge/discharge of the distributed capacitance, an oscillating current flows therethrough to generate a ringing magnetic field, resulting in raster ringing.
In order to prevent such cross-talk and hence raster ringing from occurring, 62-281242/1987 proposes to provide an electrostatic shield between the horizontal deflection coil and the vertical deflection coil and connect an element which can cut-off a horizontal deflection signal component supplied to the horizontal deflection coil in series with the vertical deflection coil.
Despite these prior art propositions, raster ringing is neither prevented nor sufficiently reduced as yet.